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Sound Waves and Pressure

Investigation 2 – Concept Day








Sound Waves and Pressure: Investigation 2

Concept Day


Note: In this Investigation, we begin with a quick review of frequency from Investigation 1. We also discuss two electrical transducers that you will be somewhat familiar with, the dynamic microphone and the electromagnetic speaker. We will use these devices, along with on-stage amplifiers, to discuss the frequency (pitch) and amplitude (loudness) of sound.

We will introduce the concept of the speed of sound and, in the process, compare it to the speed of light. This will be of benefit leading into the following CELL, Light.

Finally, we will review aspects of sound wavelength that will be useful for Investigation 2 Lab.




  • In this slide, we review the concept of sound waves and frequency from Investigation 1.
  • In addition, we refer to the tag for the 1979 sci-fi movie Alien… In space, no one can hear you scream. Space is a vacuum, it contains no molecules. Since sound waves require the vibration of nearby molecules to spread, sound cannot travel through space.
  • The picture of the singer simply leads into the next slide, where we will discuss how a microphone works.



  • This slide shows how a microphone works. This is a specific kind of microphone know as a dynamic microphone. It is the type of microphone most commonly used on stage and in concepts.
  • Begin by considering the green text concerning what an electronic transducer is and why a microphone is a transducer. Transducers convert one type or form of energy into another type or form of energy. In the case of the microphone, it converts mechanical energy caused by the moving (vibration) of the diaphragm into electrical energy.
  • The diaphragm vibrates when struck by sound waves. Another example of a diaphragm LabLearner students will be familiar with from elementary school is the diaphragm of a stethoscope. In a stethoscope, sound waves from the heart-thumping in the chest cause the diaphragm to vibrate. The vibrating diaphragm causes pressure waves in the tubing of the stethoscope to transfer the sound energy (kinetic energy) to the listener’s eardrum.
  • In the microphone, the diaphragm causes a coil of copper wire that is attached to it to vibrate. The coil is located within a magnet. When a coil of wire moves in a magnetic field, a current is produced. This current has characteristics based on the intensity and frequency of the vibrating coil and diaphragm.
  • The current produced by the vibrations is carried from the microphone by two wires leading from the coil. The current and voltage leaving the microphone is known as the audio signal. The audio signal can be recorded onto tape or digitized and saved for later playback. Alternatively, as in the case of a live performance, it can be sent to speakers.



  • A magnet speaker operates in almost the reverse manner as does a dynamic microphone. An audio signal, consisting of current and voltage, enters into a wire coil in the speaker that is located within a magnetic field. This causes the coil to vibrate at the same frequencies that had originally created the current in the microphone.
  • The coil causes the large cone to vibrate and the large vibrating surface sends sound waves into the room.
  • Thus, as shown in green text, a speaker is also a transducer. It converts electrical energy from the audio signal to the mechanical energy of sound waves – the opposite of what the microphone does.



  • In reality, the strength of the audio signal coming from a microphone is not enough to cause the vibration of large speakers to create loud sound. In practice, therefore, the audio signal from the microphone is first sent to an amplifier to be amplified.
  • In an amplifier, the incoming weak audio signal is boosted into a signal powerful enough to be sent to large speakers. In this slide, the sound wave pictured near the singer represents the audio signal. Notice it has a frequency of 440Hz (this is an A note) and a rather small amplitude. The amplitude determines how loud we will hear the sound.
  • The A-note (440Hz) signal leaves the amplifier with higher voltage and energy and reaches the speakers (see the bottom wave graph to the left of the speakers). Notice that while the frequency is identical to that of the audio signal (440Hz), the amplitude has increased. It is this increase in amplitude that an amplifier causes and is responsible for all the loud noise at a rock concert.



  • This slide highlights the difference between frequency and amplitude. As shown, frequency is responsible for the pitch we hear and is abbreviated as Hz for Hertz. Remember from Investigation 1 and the review in slide SOUND-2-2, that frequency is measured in vibrations per second. Thus, a frequency of 440 Hz would vibrate 440 times per second.
  • Amplitude is measured in decibels, abbreviated dB. It is plotted on the x-axis of the sound wave graph and is what we interpret as loudness.
  • On the right of this slide is a chart showing common sounds that occur at various decibel levels. Sounds grow louder and louder as we move up the chart. Notice that damage to the human ear can begin at about 85 dB and up. Nonetheless, we may well encounter some of the sounds above this level. Limiting exposer and wearing ear protection will minimize damage to our ears.



Note: This is the first of two slides that discusses the speed of sound and, in particular, its relation to the speed of light. This discussion is useful here as the next CELL will be on light and speed is one way to compare sound and light.

  • This baseball analogy essentially draws on the experience of being able to see an event before we can hear it. In this case, an observer is standing far up in distant centerfield, about as far as one could get from home plate (not the best seats in the house!). When we see the batter swing, we will wait nearly a half-second before we hear the “crack” of the bat. Although this seems like a short period of time, this spectator will easily perceive it as a lag.



  • This is the second of two slides that discusses the speed of sound and light. It is a much more dramatic example of the huge difference between the two speeds.
  • In practice, one can estimate the distance at which lightning strikes by counting the seconds between the flash (one one-thousand, two one-thousand, etc.) and when you hear its thunder and then dividing the number of seconds by 3. This will give you the number of kilometers away the lightning occurred. Divide by 5 instead to get miles.



Note: This slide is included simply as a review for students prior to Investigation 2 Lab. It depicts a single wavelength and indicates that the antinode, node, antinode, node distances are measured in quarters.

Note: In the lab, you will attempt to measure sound waves to their first antinodes in your experiments. You will therefore need to multiply times four (X4) to calculate the length of the entire sound wave.